Recently the long-term environmental effects of chlorofluorocarbons have come under substantial scientific scrutiny. It has been postulated that these chlorine-containing materials decompose in the stratosphere, under the influence of ultraviolet radiation, to release chlorine atoms. Chlorine atoms are theorized to undergo chemical reaction with the ozone layer in the stratosphere. This reaction could deplete or at least reduce the stratospheric ozone layer, thus permitting harmful ultraviolet radiation to penetrate the earth's protective ozone layer. A substantial reduction of the stratospheric ozone layer could have a serious deleterious impact on the quality of life on earth.
Refrigerant 22 and Refrigerant 502, the azeotropic mixture of about 47-50 weight percent HCFC-22 and 53-50 weight percent CFC-115 (the azeotrope is composed of 48.8 weight percent HCFC-22 and 51.2 weight percent CFC-115) have long been used as refrigerants in most of the country's air-conditioners, heat pumps and supermarket refrigeration cases. However, since CFC-115 is a chlorofluorocarbon compound which is being phased out by the year 2000 and HCFC-22 is a hydrochlorofluorocarbon which will be phased out in the year 2005 for new equipment, the industry is required to replace R-22 or R-502 with environmentally safer fluorinated hydrocarbons.
The tetrafluoroethanes (HFC-134 and its isomer (HCFC-134a) have been mentioned as possible substitutes. However, the low vapor pressures (relatively high boiling points) limit the refrigeration capacity of these compounds, making them undesirable in many R-22 and R-502 applications. Also, pentafluoroethane (HFC-125) has been suggested as a replacement for R-502, but its' energy efficiency (heat removed by the evaporator divided by the power to compress the vapor) is 10% lower than R-502. Consequently, newly designed equipment would be required to achieve the refrigeration currently needed for supermarket applications, cooling for air-conditioning applications and heating for heat pumps applications.
Mixtures of environmentally safe materials might also be used if the desired combination of properties could be attained in a simple (not constant boiling) mixture. However, simple mixtures create problems in the design and operation of the equipment used in refrigeration systems. These problems result primarily from component separation or segregation in the vapor and liquid phases.
Azeotropic or constant boiling mixtures of two or more components, where the composition of the vapor and liquid phases are substantially the same at the temperatures and pressures encountered in the refrigeration cycle, would appear to be the answer. Included in the definition of constant boiling mixtures are near-azeotropic mixtures. U.S. Pat. No. 4,810,403 teaches that near-azeotropic mixtures maintain a substantially constant vapor pressure even after evaporative losses, thereby exhibiting constant boiling behavior.
It is an object of the present invention to provide a constant boiling composition of at least two hydrofluorocarbons that is low boiling, is non-flammable, and suitable for use as a refrigerant, aerosol propellant, a heat transfer medium, a gaseous dielectric, a fire extinguishing agent, an expansion or blowing agent for polymers and as a power cycle working fluid.